Apple is handling the issue on their end by reissuing installers, which are signed with an updated certificate that expires on February 7, 2023. If a needed application with the correct version is still available from the MAS, the easiest fix may be to re-download the installer from the MAS. The newly-downloaded installer will be signed with the new certificate.

In the case of applications where the needed version is no longer available from the MAS, or the application itself is no longer available, there are two ways to handle this issue:

Change computer date to something before expiration to use older packages. I tested a bunch of install configs for my laptop 2 weeks ago using my last downloaded Mavericks installer, sat down to run the real install yesterday and osx installers refused to verify and run, all of them even back ups. I noticed this same cert issue and just changed time accordingly, BAM mavericks installing.